1. Field of the Invention
This invention relates to a hydraulic fluid line pressure regulator of the type used to regulate line pressure in an automatic transmission. More particularly, this invention relates to such a system controlled through actuation of a solenoid wherein the actual line pressure is controlled in an open loop system.
2. Description of the Prior Art
The line pressure in an automatic transmission is usually established by connecting the outlet of the hydraulic pump to a pressure regulator valve through which the regulated fluid is directed to selectively operated shift valves, which control the engagement of clutches and brake servos whose selective engagement produces the various speed ratios of which the transmission is capable. If line pressure is too high, the clutch elements engage too quickly producing abrupt and objectionable shifting. If line pressure is too low the clutch elements are engaged over a protracted period, which subjects the clutch elements to excessive wear and heat. Prior art transmissions of the hydraulically actuated type include hydraulic and pneumatic means for varying the line pressure, generally as a function of throttle position. Here it is proposed to regulate the line pressure with an electric solenoid that operates a line regulator valve. In this system, line pressure is varied between a minimum and maximum value according to the duty cycle of the solenoid. In the prior art the electrical solenoids used to regulate line pressure have been expensive precision variable force solenoids. The control systems in the prior art use a closed loop whereby information concerning the magnitude of variations in the regulator line pressure is sensed and applied as input in the form of an error signal.